1. Field of the Invention
The present invention relates to methods of the manufacture and repair of turbine blades and is primarily concerned with the manufacture and repair of turbine blades that are provided with a durable portion adjacent the outer end of the leading edge.
During operation of a turbine, the blades are subject to several hazzards. The primary hazzard affecting the leading edge of the blades is impact with:
(a) solid particles
(b) water droplets.
Impact with solid particles and water droplets causes erosion to occur in the blade, solid particle erosion occurs in view of solid particles, metallic or chemical, carried over from the boiler, for example, coming into contact with the leading edge of blades situated normally at the early stages of the turbine.
Impact with water droplets can also cause considerable damage to the turbine blades and is more prone to occur in later stages of the turbine. The reason that water droplet damage occurs in the later stages of the turbine is that the steam has cooled compared with its temperature in the high pressure stages and water condenses more readily. The combination of the chemicals in the water and the stresses to which the blades are subjected in use can lead to an effect known as stress corosion cracking, particularly if there is a flaw or transitional weakness present in the blade.
Dense moisture forms in the expanding steam and is transported by the steam. Water particles are, after deposition on portions of the turbine re-entrained with a main steam flow and can collide with the leading edge of the moving blades.
In the case where water is heated by nuclear energy, the temperature of the water is lower than would be the case with, for example, a fossil fuel generating plant and hence the blades in a turbine are more prone to water droplet impact and the aforementioned effect of stress corosion cracking.
Impact with water droplets can cause quite severe damage, there are two effects of the impact that can account for the erosion that takes place:
(1) initially a high pressure intensity between the droplet and the material of the blade:
(2) the high radial flow velocity of the water contained in the droplet after collision.
In order to combat such erosion it is well known to fit a nose or shield of hard material, for example Stellite, to the leading edge of the blades. However the material itself is very expensive and unless perfectly fitted can lead to further problems.
A shield may be attached to a blade by what is known as peripheral welding, peripheral welding can leave a junction of the shield at the radially outermost position where no weld material is present and a cavity or thin gap can exist under the shield. If water penetrates this gap considerable damage can occur, including delamination of the shield from the blade and/or the formation of cracks in the blade.
A Stellite shield may be attached to a blade by welding or by brazing. Welding provides for a stronger or better attachment of the shield to the blade particularly where the shield is in a form of a "nose" of the blade.
When a Stellite shield is attached to a blade by brazing, if there are any voids in the braze the shield may easily become dislodged due to the flexing of the blade during operation with consequential damage to adjacent blades and/or tubes in the condensor which follows the low pressure stage of a turbine.
Additionally, when the braze is onto nickel (where a part of the blade has been nickel plated) the braze and nickel plate can become delaminated due to water entering the void by capillary action and subsequent deliterious affects to the joint.
The heat generated during the welding of the Stellite shield to the blade makes it essential for critically controlled preheat and subsequent heat treatment of the blade after the welding operation to minimise any concentrations of stress. It is usually recommended that such repairs are carried out after the blade has been removed from the rotor under very carefully controlled conditions.
2. Prior Art
Japanese patent application 1969 162010 discloses a turbine blade in which a piece of the leading edge of the blade at the outer end is removed and a piece of material the same as that of the parent material of the blade is then welded to the blade and subsequently hardened by flame hardening. The above mentioned Japanese patent application mentions specifically a 12% chrome steel as one of the steels from which the blade may be made. However, the carbon content of the steel is not mentioned. If the carbon content is relatively low, for example 0.1%, then the hardening achieved by flame hardening may be insufficient to provide any real protection against erosion of the blade during use.
If the blade and insert is made of a higher carbon material, then a greater degree of hardness may be achieved. However it is undesirable to manufacture turbine blades, in particular turbine blades used in the low pressure or wet end of a turbine, from a hard material.
There are however considerable problems in producing the blade as a whole from hard material in that the blade is considerably less ductile than a blade made from the more normal 12% chrome steel with fairly low carbon content. A high carbon content in the blade can increase the chances of the occurrence of stress corosion cracking, particularly where a blade is at a low pressure or wet end of the turbine and in the regions of the blades where a discontinuity, such as lacing wire holes or cover band slots occur. This problem has been recognised with the result that the blade is sometimes thickened in such areas. This obviously leads to greater expense in manufacture and a reduction in the "throat gap" between one blade and another with consequential loss of efficiency.
Since the material of the insert is substantially the same as the material of the blade, if cracks occur to the leading edge due to impact with some foreign material, then the crack can develop across the blade in view of the hardness of the material as a whole.
The normal material, e.g. 12% chrome steel, from which turbine blades are commonly made, in particular those in the low pressure end of a steam turbine, is not a particularly hard material since the blades have to be provided with discontinuities such as lacing wire holes, whereby the blades in a rotor may be joined together with a lacing wire. If a hard material was used, the presence of such discontinuities could easily lead to the formation of cracks in use of the turbine and hence a comparatively short service life.
British patent application 8630502 (Publication No. 2,198,667), which corresponds to U.S. Pat. No. 4,832,252, issued May 23, 1989, discloses a method of repairing a turbine blade in which a piece of material is removed from the blade and an insert is secured thereto by welding, the insert being made from a material which has a part compatible with that of the material of the turbine blade and another part of high durability which forms the leading edge of the turbine blade once the insert is secured thereto.
Where the insert can be prepared under controlled conditions a highly satisfactory result can be achieved providing care is taken during securing of the insert to the blade and in subsequent heat treatment of the area affected by the weld.